index-pack: run setup_git_directory_gently() sooner

index-pack: run setup_git_directory_gently() sooner

index-pack already runs a repository search unconditionally; running
such a search earlier is not risky and ensures GIT_DIR will be set
correctly if the configuration needs to be accessed from
run_builtin().

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>